Inclusion Complexes of Cyclodextrins with Methylene Blue and Acid Orange 7 in Aqueous Solutions

Abstract

Abstract Upon the addition of Acid Orange 7 (AO7) to aqueous Methylene Blue (MB) solution buffered at pH 5.0, the absorption spectrum of MB is reduced in intensity, accompanied by a red shift of the absorption peak, indicating the formation of a complex between MB and AO7. From a simulation concerning the fluorescence intensity change of MB, the equilibrium constant for the formation of the 1 : 1 MB-AO7 complex has been estimated to be 66300 mol-1 dm3. γ-Cyclodextrin (γ-CD) forms a 1 : 2 and a 1 : 1 inclusion complex with MB and AO7, respectively. From the absorbance changes, equilibrium constants for the formation of the 1 : 2 γ-CD-MB and 1 : 1 γ-CD-AO7 inclusion complexes have been evaluated to be 2.95×107 mol-2 dm6 and 11000±300 mol-1 dm3, respectively. When γ-CD is added to MB solution containing AO7, the absorption band of MB is enhanced in intensity without an appearance of the MB dimer band, in contrast to MB solution without AO7. This finding indicates the formation of a ternary γ-CD-MB-AO7 inclusion complex. To determine the stoichiometry of the ternary inclusion complex, a continuous variation method using the absorption and fluorescence intensities has been performed under the conditions that the sum of the initial concentrations of γ-CD and AO7 has been kept constant. Both the results indicate the formation of a 2 : 1 : 1 γ-CD-MB-AO7 inclusion complex. The equilibrium constant for the formation of the ternary inclusion complex from γ-CD and the 1 : 1 MB-AO7 inclusion complex has been evaluated to be 3.40×1010 mol-2 dm6 from the fluorescence intensity change.